HEATING
(comprehensive)
HIGH
FREQUENCY HEATING
In
conventional processes, used in the field of drying and bonding of wood, due to
the poor heat conduction of the wood, the heating processes require a lot of
time. With an increasing thickness of the wood, the time required for the
transfer of heat to the target position (such as the bonding juncture) will also
increase, resulting in long processing time. Complete heating-up of the carrier
material will also take place, which may be undesirable for reasons of fast
subsequent treatment.
The
material is heated up from the outside to the inside. There is a great
disadvantage, because more or less volumes of wood must also be heated up before
the required amount of heat may be transferred to a bonding juncture. The
opposite is true with the application of high frequency (HF). This technology
has been used for decades, resulting in a considerable reduction of the
processing times in various fields of production.
The
initial position is quite different with the application of high frequency (HF).
The high frequency energy is transferred to heat directly within the wood, so we
can say that the wood is heated from the inside to the surface.
The Principles of High Frequency Technology
By
using high frequency technology, for instance for bonding purposes, the wood or
the carrier material and the bonding agent are exposed to a capacitive or
dielectric heating process in an alternating voltage high frequency field.
In
simple terms: the smallest components of water present in the wood, the
molecules, behave similar to small magnets in a magnetic field. The molecules
are subjected to high frequency oscillations, resulting in the generation of
heat due to friction caused by mutual obstruction.
The
bonding juncture is heated up directly with higher intensity and faster than the
surrounding wood when the high frequency alternating voltage is applied, because
it represents considerably higher dielectric losses. This phenomenon is also
called "selective heating".
Water
molecules, the so-called dipoles, are increasingly deflected from their at-rest
position with closer proximity of the voltage to the electrodes. It is easily
understandable, that a greater amount of friction work is performed with an
increase of the distance - whereby the generation of heat will increase by a
square-law function with the voltage applied. That is, if the voltage applied is
doubled, the generation of heat will be four times as high.
The
Physical Context
It
is not without consequence, whether a specific voltage is applied to a capacitor
with plates separated by 3 cm or by 10 cm. For this reason, the voltage U (V)
must always be referenced to the distance d (cm) in order to clearly define each
case. This is performed in a homogenous field by dividing the voltage U applied
to the electrode by the distance d between the electrodes. The resulting voltage
per cm (V/cm) thus obtained is the field strength E.
The field strength, however, may not be increased at will, because otherwise
flashovers or burning of wood might take place.
Some
further important Factors for determining how fast and well the wood may be
heated are the so-called material-dependent dielectric constant epsilon and the
loss factor tangent delta.
The dielectric constant epsilon tells how many times the capacity of a capacitor
with a dielectric is larger than an air capacitor with the same dimensions. The
loss factor may be compared with the power factor cosine phi in heavy current
technology. It indicates the percentage of the power converted into heat, and
therefore serves as an indication of the actual "heat yield" from the
electrical energy applied.
It
may generally be assumed that the dielectric constant epsilon at a specific
frequency will increase slightly with an increasing degree of humidity, while
the loss factor tangent delta will remain approximately proportional to the
content of water.
Special advantage is taken of the fact during bonding with high frequency, that
bonding agents feature loss factors up to 40 times higher than wood. The bonding
junction may thus be heated up with a minimum of energy. Bonding times of
several hours (in case of cold bonding) may thus be reduced to several minutes
only.
Advantages of High Frequency Technology
The
most important advantages of high frequency heating are summarized as follows:
Areas
of Application for High Frequency Procedures
The
high frequency process is not a new technology. It has already been used in
woodwork and wood processing for several decades. Above all only drying and
bonding of wood have been of importance up to a while ago. But some other areas
as well, such as the drying of water lacquers, seem to be strictly predestined
for the application of high frequency. It may be expected, that this technology
will become increasingly important in respect to the rationalization and
reduction of manufacturing processes.
High
frequency technology is preferably used in the area of wood drying, if the goods
to be dried are components which are thick, preformed and do not comprise
excessive humidity. Opposed to the convection drying process, drying is here
performed from the inside to the outside. Excessive drying speed might therefore
result in an explosion.
High
frequency is also used for the bonding of wood, either as a continuous or as a
stationary procedure. Broadside, longitudinal and surface area bonding (as with
the manufacturing of parquetry, solid wood panels or glue bonding) may also be
mentioned in this context. High frequency treatment has not been very important
in the lamination of window frames , but there may be potential applications
ahead in this area.
An
abbreviated processing time may be achieved with high frequency heating in the
production of particle and MDF boards.
A
field of application which is still very young, but which seems to have a
promising future, is the drying of water lacquers. A Hessian manufacturer of
stairs has commissioned such a plant only last year, which is the first one
worldwide. The drying and processing times could be reduced extremely, and the
required manufacturing area was reduced to a minimum.
INDUCTION
HEATING
Induction
heating is a method of heating conductive material by subjecting it to an
alternating electromagnetic field, usually at frequencies between 100 and 500
kHz.
An
inductor (the work coil), acting as a primary winding of a transformer,
surrounds the material which is to be heated (the work piece), which acts as the
secondary winding. Alternating (RF) current flowing in the primary coil induces
eddy currents in the work piece and heats it up. The frequency of the primary
alternating current, along with the permeability and resisitivity of the
material, decide the depth that the eddy currents penetrate and therefore the
distribution of heat within the work piece. The particular design of the coils,
along with temperature sensors and feedback controls, allows either the entire
work piece or a specific area to be heated. The repeatability of the process is
excellent.
Oscillator
circuits containing triodes are commonly used to generate the RF currents.
Applications
of Induction Heating
Pipe
Welding
Induction
welding of tube and pipe products involves the formation of a metal strip
between specially designed rollers. The seam is brought together under a small
amount of pressure and electric current induced along the seam to cause the
welding.
Triodes
are used in oscillator circuits to feed RF current to the pipe welding work
coil, which induces eddy currents in the seam being welded.
Induction
Hardening/Heat Treatment
Inductive
heating is used for heating precisely predetermined areas of electrically
conductive materials, such as steel and other metals, to give particular
hardness or strength properties.
The
process demonstrates excellent repeatability. The required surface hardening,
for example, can be pre-set to exactly the necessary depth. All parts in the
following production batch will have the same characteristics.
As
in pipe welding, triodes and tetrodes are used in oscillator circuits to feed RF
current to a work coil, inducing eddy currents in the component and thereby
heating it rapidly.
Dielectric
heating (also known as Capacitance heating) is the method of heating
non-conductive materials. The material to be heated is placed between two
electrodes, to which a high-frequency energy source is connected. The
oscillating field passes through the material and as the field direction
changes, the polarisation of individual molecules reverses rapidly, causing
friction and hence heat. The higher the frequency, the greater the movement.
Typically, frequencies in the range 5 MHz to 80 MHz are used.
Scientifically
speaking, there are several ways that a dielectric material absorbs energy from
the oscillating electric field. The two most important mechanisms are
molecular rotation and electrical conduction. Some dielectric materials'
ability to conduct electricity (moderate resistivity) is good enough that
an amount of RF or microwave current will flow and heat the material. This
mechanism is especially important at lower frequencies and with
semiconducting materials.
The
other mechanism, molecular rotation, occurs in materials with polar molecules.
A material capable of being heated with RF or microwave energy is said to be
polar, referring to the fact that its molecules have both positive and negative
opposing charges (dipolar). In practical application an electric field is
applied to the material causing its molecules to rotate and line up with their
corresponding fields. RF and microwave energy fields alternate much like
an electric motor, between positive and negative, at their specified frequency
of operation thus causing the molecules of the material to rotate. The
friction generated by the molecules rubbing together as they rotate generates
heat. This method of generating heat within a material is termed dipole
rotation and can be used to heat solids, liquids, or gases
Wood Gluing
Plywood,
laminated wood, chipboard and MDF are examples of glued-wood products. These
products are characterised by high density, improved dimensional stability,
strength and appearance and therefore they have a greater advantage over natural
wood.
The
main advantage of using RF for wood gluing is that the adhesive setting time is
reduced from hours to minutes. This has enabled mass production to become open
to the market.
RF
Drying
Many
industries use a drying process such as food preservation (complete removal of
moisture immediately prior to packaging) and textiles (removal of water from
freshly dyed bobbins of textile). RF drying is ideal for many drying
applications as water is very receptive to dielectric heating
Plastic Welding
Using
principles similar to those employed with metals, some plastics can be joined by
welding (e.g. PVC, ductwork and polyethylene tanks). Surfaces are joined by
heating the plastic joint under pressure. RF energy is easy to control and is
ideal for plastic welding as the areas to be heated can be localised while the
rest of the material remains cool.
Typical
products produced using RF plastic welding are office stationery, inflatable
boats, tarpaulins and medical supplies.